The current study investigated comorbidity in eighty-nine children and adolescents with Autism Spectrum Disorder in Ireland. Comorbidity is the presence of one or more disorders in addition to a primary disorder. The prevalence of comorbid psychological disorders, behaviours associated with comorbid psychopathology, epilepsy, gastrointestinal symptoms and sleep problems were examined. Age, gender, level of intellectual disability, presence of epilepsy, attention deficit/hyperactivity disorder (AD/HD) and an anxiety disorder were determined using a self-constructed demographic questionnaire. The Autism Spectrum Disorder-Comorbidity-Child (ASD-CC) was administered to informants to assess symptoms of psychopathology and emotional difficulties. The Children's Sleep Habits Questionnaire (CSHQ) and Gastrointestinal Symptom Inventory were administered to assess sleep problems and gastrointestinal symptoms respectively. Forty-six percent of participants had a comorbid disorder, with this number increasing to 78.7% if intellectual disability was included. The prevalence of epilepsy was 10.1%, AD/HD was 18% and an anxiety disorder was 15.7%. Prevalence rates of gastrointestinal symptoms and sleep problems are discussed in the study. (Contains 6 tables.)